Immunoglobulin transport across the small intestine of the neonatal rat and the yolk-sac of the fetal rabbit will be investigated as models for selective protein transport by cells. In the neonatal rat, IgG immunoglobulins have been shown to be transported across the epithelial cells in the proximal small intestine by a process of selective pinocytosis. Preliminary studies have demonstrated that the Fc portion of the immunoglobulin selectively binds to the surface of membrane invaginations at the base of microvilli. The proposed research will compare the transport of whole immunoglobulins with that of immunoglobulin fragments by conjugation of these proteins to horseradish peroxidase as a histochemical marker for electron microscopy. Membrane binding and subsequent steps in the transport of the tracers will be compared at different pHs and protein concentrations. Membrane binding and initiation of pinocytosis will be correlated by measuring the number of pinocytotic vesicles formed at the different tracer concentrations. In addition, selective binding sites will be isolated from purified microvillus membranes of epithelial cells. Detergent-solubilized membrane components will be subjected to polyacrylamide gel electrophoresis in order to identify the binding sites and to determine their molecular weight. In the pregnant rabbit, maternal antibodies are selectively transferred to the fetus across the yolk-sac instead of the gut. Peroxidase-labelled immunoglobulins and antiperoxidase antibodies will be injected into the uterus of pregnant rabbits to determine the morphological site and intracellular route of transport within the yolk- sac endoderm.